The invention relates to a method for producing electrically conductive circuits on a print base board having a copper lamination attached on one side face thereof, and more particularly relates to such method for producing the circuits of excellent electrical conductivity by effectively utilizing a newly developed electrically conductive copper paste which is specifically adapted to metal plating, wherein at least two-layer circuits are formed on one side of the base board, of which a first-layer circuit having a plurality of electrodes and a second-layer circuit having a plurality of ring-shaped electrodes formed in connection with the electrodes of the first-layer circuits, each electrode of the second-layer circuit being defined by outer and inner circumferences with a central opening defined by the inner circumference, such that the metal plating applied to the electrodes of the first- and second-layer circuits with a predetermined thickness of the metal plating layer may provide an electrically conductive path between the two circuits including an elongated total length of the metal plating layer connected to the first-layer circuit, the metal plating layer having an enlarged sectional area to increase the electrical conductivity and to increase the adherence force of the second-layer circuit to the first-layer circuit.
It is generally known that the more or less complex circuits formed on a base board have some portions to be electrically connected to each other. So far it has been industrially difficult to form more than two-layer circuits on one side of the base board, and therefore the circuits have been formed, one on one side face of the base board and another on the opposite side face of the base board by etching the copper lamination attached to each side of the base board, and then the two circuits have been electrically connected to each other by means of a through-hole extended vertically of the base board.
According to such a conventional method, it is required to attach copper laminations on both sides of the base board and to etch the copper laminations to form circuits thereon respectively and then to form many through-holes in the base board by way of the NC device. The production cost is therefore considerably high including the costs for the materials to be used and the processing steps, and moreover the production efficiency is lower.
It is therefore desired to form more than two-layer circuits on one side face of the base board. To satisfy this requirement, it has been needed to develop an electrically conductive paste which is of lower cost and excellent in electrical conductivity and further specifically adapted to a metal plating such as a copper plating. The development of such an electrically conductive copper paste has been difficult because copper is very easily oxidized by heat in contrast to the precious metals such as gold resulting in increasing the electric resistance and deteriorating the soldering effect. Further in order to apply a metal plating to the heated and hardened electrically conductive copper paste, it has been required to activate the surface of the copper paste by means of a catalyst so as to depose the copper particles to the surface of the copper paste from among the binders of a resin layer contained therein, which copper particles being the nucleuses for making effect the subsequent metal plating.
The Japanese Utility Model application (laid open No. 55-42460) discloses a method for forming more than two-layer circuits on one side face of a base board, wherein a high isolation-resistant polybutadiene is employed for formation of an isolation film, and the base circuit is covered with a copper film in such a manner that an adhesive paste of 20% of phenol resin, 63% of copper particles and 17% of solvent is coated on the base circuit, and then a non-electrolytic plating is applied to the adhesive paste to form a plating film up to 20 .mu.m thereon. Such a method has never been reduced to practice due to the demerits as mentioned above.
The applicant has continued for many years to work for development of electrically conductive copper pastes which may eliminate the defects and disadvantages of the prior art, and succeeded in realization of such copper pastes to be industrially utilized. These are the electrically conductive copper pastes ACP-020, ACP-030 and ACP-007P developed by Asahi Chemical Research Laboratory Co., Ltd. which are substantially composed of copper particles, synthetic resins and a small amount of anthracene as a specific additive by way of example.
The copper paste APC-020 is substantially composed of 80% by weight of copper particles and 20% by weight of synthetic resins, and is excellent in electric conductivity, but not so good in the soldering property. The copper paste ACP-030 is substantially composed of 85% by weight of copper particles and 15% by weight of synthetic resins, and is not so good as the copper paste ACP-020 in electric conductivity but excellent in the soldering property. The copper paste ACP-007P is an improvement of the copper paste ACP-030 which may be subject to a metal plating such as a chemical copper plating to be applied to the hardened surface thereof without the need of catalyst treatment, and is extremely excellent in the metal plating property.
According to the invention, the electrically conductive copper paste ACP-007P is employed. The copper paste is coated on the first-layer circuit which may be subject to a metal plating and then is heated to be hardened. Subsequently a metal plating is applied to the copper paste to form the second-layer circuit on the first-layer circuit, the former being electrically connected to the latter, to thereby form at least two-layer circuits on one side face of base board.
In fact, this method is disclosed in the Japanese patent application No. 60-216041 (corresponding to U.S. Pat. No. 4,734,156). According to the method, as shown in FIGS. 15 and 16, a first-layer circuit C.sub.1 is formed on a copper lamination 2 attached to one side face of a base board 1 by etching the copper lamination. Then a plating-resistant resist 3 is coated by way of printing all over the base board except the portions 2a,2b of the first-layer circuit which are to be electrically connected to an additional circuit to be formed on the first-layer circuit, though the resist 3 is not seen in FIG. 15 because the resist is transparent. Subsequently the electrically conductive copper paste 4 (ACP-007P) which is specifically adapted to a metal plating is coated by way of a screen printing on the portions which remain without the resist being coated thereon, and is then heated to be hardened and is washed to be clean. Then a chemical copper plating by way of example as a metal plating is applied to the electrically conductive copper paste 4 to form a copper plating layer 5 thereon to thereby provide the second-layer circuit C.sub.2 of the copper plating layer and the electrically conductive copper paste 4. Thus at least two-layer circuits C.sub.1, C.sub.2 are formed on one side face of the base board.
However according to this method, as shown in FIG. 15, the electrically conductive copper paste 4 coated on each electrode 2d of the first-layer circuit C.sub.1 is in a shape of a bar with a predetermined width having an end 4a defined by an arc. In this connection, the electric conductivity of the copper plating layer 5 with respect to the electrode 2d depends on a total area of the thickness 5a of the copper plating layer 5, that is a dotted area in FIG. 15, which is connected to the electrode 2d of the first-layer circuit C.sub.1. In short, the conductivity depends on the dimensions the sectional area of the electrically conductive path provided between the first- and second-layer circuits C.sub.1,C.sub.2. Further the adhesion force of the second-layer circuit C.sub.2 to the first-layer circuit C.sub.1 depends on the area of each electrode 4d of the second-layer circuit C.sub.2 including the outer boundary 4e of the electrode 4d. It is therefore apparent that the shape of the electrode 4 d of the second-layer circuit C.sub.2 is not sufficient to provide a desired electric conductivity and a desired adherence force of the second-layer circuit C.sub.2 to the first-layer circuit C.sub.1.